Increasing the detonation pressure of ammonium nitrate/fuel oil compositions

ABSTRACT

THE DETONATION PRESSURE OF A BULK AMMONIUM NITRATE/ FUEL OIL (ANFOO) COMPOSITION IS INCREASED SUBSTANTIALLY INDEPENDENTLY OF ITS DETONATION VELOCITY BY THE ADDITION OF IRON OXIDE, CALCIUM CARBONATE, AND/OR CALCIUM SULFATE AS A DENSIFYING AGENT TO PRODUCE A MIXTURE CONTAINING ABOUT 8-30% DENSIFYING AGENT. A DETONATING COMPOSITION CONTAINING A BLEND OF COARSE AND FINE AN, FUEL OIL, AND IRON OXIDE, CALCIUM CARBONATE, AND/OR CALCIUM SULFATE, AND A PROCESS FOR SHOCK-SYNTHESIZING DIAMOND BY USE OF THE DETONATING COMPOSITION.

United States Patent 3,823,044 INCREASING THE DETONATION PRESSURE OF AMMONIUM NITRATE/FUEL OIL COMPOSITIONS George R. Cowan, Woodbury, N..I., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del. No Drawing. Filed Aug. 30, 1972, Ser. No. 284,915 Int. Cl. C06b 1/04 US. Cl. 14921 7 Claims ABSTRACT OF THE DISCLOSURE The detonation pressure of a bulk ammonium nitrate/ fuel oil (ANFO) composition is increased substantially independently of its detonation velocity by the addition of iron oxide, calcium carbonate, and/ or calcium sulfate as a densifying agent to produce a mixture containing about 830% densifying agent. A detonating composition containing a blend of coarse and fine AN, fuel oil, and iron oxide, calcium carbonate, and/or calcium sulfate, and a process for shock-synthesizing diamond by use of the detonating composition.

BACKGROUND OF THE INVENTION This invention relates to a method of increasing the detonation pressure of an ammonium nitrate/ fuel oil composition substantially independently of its detonation velocity; an ammonium nitrate/ fuel oil composition of high detonation pressure and moderately high detonation velocity; and a diamond shock synthesis process employing the high pressure, moderately high-detonation-velocity composition.

Mixtures of prilled ammonium nitrate (AN) and fuel oil, commonly referred to as ANFO mixtures, have been widely accepted as commercial blasting agents owing to their low cost, safety, and ease of loading. However, these blasting agents suffer from the serious disadvantage of excessively low bulk, (i.e., loading or poured) density that leads to low detonation pressure and consequently low explosive performance or work potential. Inasmuch as the detonation pressure of an explosive or blasting agent is approximately proportional to the product of the density of the explosive and the square of the detonation velocity, the detonation pressure of a given composition can be increased by increasing its density. This can be accomplished in the case of an ANFO mixture by using an appropriate blend of standard AN prills with denser, smaller AN particles, instead of AN prills alone, as the AN component of the mixture. This method of increasing the detonation pressure of an ANFO mixture, however, cannot be employed to increase the pressure independently of the detonation velocity, inasmuch as the pressure increase is accompanied by an increase in detonation velocity.

The addition of foreign substances such as certain fuels, barium sulfate, sodium chloride, sodium nitrate, and oxalates and formates to ANFO mixtures has been reported to increase their bulk density, but these materials have been found to cause a concomitant change in detonation velocity. In some instances, the detonation velocity has been decreased by the addition, which effect would defeat the purpose of the addition if the goal were an increase in detonation pressure, in view of the fact that a decrease in detonation velocity would either cause a decrease in detonation pressure or restrict the amount of pressure increase attainable as a result of increased density.

The detonation pressure and the detonation velocity of an explosive or blasting agent both influence the nature of the results achieved by the detonation process, e.g., the type of fracture and shattering elfect produced in blasting, the type of transformation produced in shocked materials, etc. For this reason, it is important to be able to control each of these properties independently of one another. In rock blasting, for example, it may be desirable to control the detonation velocity of the detonating composition in relation to the sound velocity of the rock. In such a case, if a selected ANFO composition has the desired detonation velocity but too low a detonation pressure, a means is needed to increase the pressure without significantly increasing or decreasing the velocity.

In the shock synthesis of diamond from non-diamond carbon by the proces described in US. Pat. 3,667,911, issued June 6, 1972, to A. S. Balchan and G. R. Cowan, a spanning or transverse shock wave is introduced into non-diamond carbon positioned in a metal container cylinder, by the progressive collision of the container cylinder and a surrounding coaxial metal driver cylinder, which is propelled by a detonation, the shock wave moving axially through the non-diamond carbon at a velocity equal to the collision velocity, or the detonation velocity of the explosive surrounding the driver cylinder. The detonation velocity preferably i in the range of about from 3000 to 6000 meters per second, most preferably about from 4000 to 5000 meters per second. At the same time, to achieve optimum yields, the detonation pressure of the explosive needs to be sufficiently high to accelerate the driver cylinder over the short distance between colliding cylinders at a sufliciently high velocity to produce the cylinder-spanning shock wave. Although the desirability of employing ANFO mixtures in a process of this kind is great when factors such as safety, economy, and convenience are considered, the ANFO mixtures heretofore known which detonated in the required velocity range did not develop a sutficiently high detonation pressure to afford optimum yields in the process. Heretofore, higher detonation pressure could be achieved only at the expense of increasing the detonation velocity to a value outside the required range.

SUMMARY OF THE INVENTION This invention provides a method of increasing the detonation pressure of a bulk ammonium nitrate/fuel oil (ANFO) composition substantially independently of its detonation velocity which comprises mixing (a) an ammonium nitrate/fuel oil composition comprised of about from 92 to 95 percent by weight of ammonium nitrate and about from 8 to 5 percent by weight of fuel oil and containing at least about 20 percent by weight of its ammonium nitrate in the form of prills, and (b) about from 8 to 30 percent by weight, based on the resulting mixture, of a particulate densifying agent selected from the group consisting of iron oxide, limestone, and anhydrite.

Preferably, the densifying agent is iron oxide and the ammonium nitrate is a blend of at least about 20 percent by weight of +20 mesh prills and at least about 20 percent by weight of a smaller-size grade which is up to about one-half, and most preferably up to about one-fifth, the size of the prills, the blend generally comprising about from 20 to percent by weight of the prills and about from 80 to 20 percent by weight of the finer grade. Most preferably, the blend comprises about from 40 to 70 percent of the prills and about from 60 to 30 percent of the finer grade. The particles of densifying agent preferably are smaller than the AN prills.

The size grade is the nominal size designation of a particulate material, each grade covering a nominal particle size range.

Also provided by this invention is an ANFO detonating composition comprising, in mixture, (a) about from 64 to 87 percent by weight of discrete particles of ammonium nitrate in a larger-size and a smaller-size grade, the larger-size grade being defined by ,-l20 mesh prills and constituting at least about 20 percent by weight of the ammonium nitrate particles in the mixture, the smaller-size grade being up to about one-half, and preferably up to about one-fifth, the size of the prills and constituting at least about 20 percent by weight of the ammonium nitrate in the mixture; (b) about from 8 to 30 percent by weight of discrete particles of a densifying agent selected from the group consisting of iron oxide, limestone, and anhydrite, the size grade of the particles of densifying agent being up to about one-half the prill size; and (c) about from 3.5 to 8 percent by weight of fuel oil, the fuel oil weight being about from 5 to 8 percent of the combined ammonium nitrate and fuel oil weight. The preferred ammonium nitrate component is a blend of about -80 percent prills and 80-20 percent finer grade, most preferably 40-70 percent prills and 60- 30 percent finer grade.

The ANFO detonating composition of this invention, which has a bulk density of at least about 1.00 gram per cubic centimeter, has a density and a detonation pressure which are higher than the density and detonation pressure of an ANFO composition having about the same detonation velocity and ammonium nitrate/ fuel oil weight ratio, and containing no densifying agent. The composition of the invention meets the requirements for use in the shock synthesis of diamond according to the process described in the aformentioned US. Pat. 3,667,911, and the present invention also provides an improvement in a process for synthesizing diamond by introducing a shock wave into non-diamond carbon by the progressive collision of a metal cylinder which contains the non-diamond carbon and a surrounding coaxial metal driver cylinder propelled by a detonation, the improvement comprising propelling the driver cylinder by means of the detonation of a mixture comprising (a) about from 64 to 87 percent by weight of discrete particles of ammonium nitrate in a larger-size and a smaller-size grade, the larger-size grade being defined by +20 mesh prills and constituting at least about 20 percent by weight of the ammonia nitrate particles in the mixture, the smaller-size grade being up to about one-half, and preferably up to about one-fifth, the size of the prills and constituting at least about 20' percent by weight of the ammonium nitrate in the mixture; (b) about from 8 to 30 percent by weight of discrete particles of a densifying agent selected from the group consisting of iron oxide, limestone, and anhydrite, the size grade of the particles of densifying agent being up to about onehalf the prill size; and (c) about from 3.5 to 8 percent by weight of fuel oil, the fuel oil weight being about from 5 to 8 percent of the combined ammonium nitrate and fuel oil weight.

DETAILED DESCRIPTION OF THE INVENTION The ANFO detonation-pressure-increasing process of this invention is applied to a bulk ANFO composition, i.e., a mass of discrete particles, as distinguished from a consolidated, self-supporting mass, e.g., a pelletized composition. The AN particulate structure of the composition can vary but basically the AN component is comprised of at least about 20 percent by weight of standard (porous) prills so as to assure the degree of oil absorption needed to avoid excessively low detonation velocities. An all-prill AN component can be employed, and the size and density of the prills varied as required. Alternatively, the prills can be blended with other forms of AN, e.g., grained AN made by the crystallization method, or with crushed prills. Mixtures of coarse and fine AN are preferred as such mixtures have higher densities, and therefore higher detonation velocities and pressures than uniform-size prills. A composition having the requisite density and detonation velocity for use in the diamond synthesis process of the invention and in other applications contains a blend of coarse and fine AN, at least about 20 percent by weight of the AN being +20 mesh (i.e., larger than 20 mesh) prills, and at least about 20 percent being of a size grade which is no more than about one-half, and preferably no more than about onefi-fth, the prill size. Generally, the prills are 8 mesh (i.e., smaller than 8 mesh). Intermediate size grades also can be present provided at least about 20 percent by weight of the coarse, and at least about 20 percent by weight of the fine, grade are present. Practically speaking, a two-grade blend is more useful and a blend of 2080% coarse and -20% fine is therefore preferred, a blend of 40-70% coarse and 60-30% fine being especially preferred. ANFO made from AN blends having more than about 65 percent of the fine grade is usually cap-sensitive at the poured density of the mixture. 1

The fuel oil content of the ANFO composition to which the present detonation-pressure-increasing process is applied is about from 5 to 8 percent, i.e., the composition ranges from about /5 to 92/8 AN/fuel oil, as the AN content decreases from about 95 to about 92 percent.

The AN and fuel oil are mixed with particulate iron oxide, limestone (or other source of calcium carbonate), and/ or anhydrite (anhydrous calcium sulfate) densifying agent in an amount such as to provide a densifying agent content of about from 8 to 30, preferably 10 to 20, percent by weight in the resulting mixture. Compositions containing less than about 8 percent densifying agent fail to show a significantly higher detonation pressure than the ANFO composition without the densifying agent. As the densifying agent content increases, e.g., above about 30%, the detonability of the composition tends to become less reliable. Also, with higher densifier contents, higher densities are required if the detonation velocity is to be kept from dropping. Therefore, densifier contents much above about 20% are not preferred inasmuch as, at these levels, the densities required to maintain the detonation velocity may be difiicult to achieve.

The aforementioned densifying agents are ideally suited for use in the present process, inasmuch as they all pro duce an increase in the detonation pressure of an ANFO mixture with no significant change in the detonation velocity thereof, i.e., a change of more than about l00200 meters per second; are relatively inexpensive; and are inert, therefore producing no charge in the oxygen balance of the ANFO. On the basis of density and economic factors, iron oxide is preferred. Any oxide of iron can be used, e.g., Fe O Fe O and/or FeO.

The densifier, which in the present process is mixed with AN and fuel oil, preferably is of a size grade which is no more than about one-half the AN particle size, or the size of the coarsest AN, e.g., the prills, in AN particlesize blends. This disparity in sizes between AN and densifier affords a mixture of higher density and better dispersion of the inert component in the mixture. A densifier particle size of about one-fifth to one-tenth, or less, of the prill size is especially preferred. Within the specified limits for densifier content, finer grades of densifier may be used in smaller amounts. For example, when a 250 mesh, high-purity magnetic Fe O (all particles pass through a 100-mesh screen, and areheld on a 250-mesh screen) is employed, a densifier content of about from 14 to 17 percent by weight is particularly preferred; whereas with a crude Fe O ore (containing -8% impurities) in which all particles pass through a ZOO-mesh screen and 15% pass through a 325-mcsh screen, a densifier content of about from 10 to 14 percent by weight is particularly preferred.

Although the order of addition of the ingredients to produce the ANFO mixture of increased detonation pressure is not critical, for uniformity of action it s preferred that AN, densifier, and fuel oil be pro-mixed prior to loading in a container or borehole. Generally, the densifier will be added to the AN and fuel oil and the ingredients mixed, or the fuel oil will be added to pre-mixed AN and densifier.

Mixing of the iron oxide, limestone, and/or anhydrite densifier with AN and fuel oil of the aforementioned composition ratio produces a mixture wherein the AN content is about 64-87 percent, the densifier content about 8-30 percent, and the fuel oil content about 3.5-8 percent (all by weight). A preferred mixture comprises about 73- 86 percent AN, about 10-20 percent densifier, and about 4-7 percent fuel oil. Minor amounts, e.g., about 1-3 percent, of other ingredients such as oil absorbents and antisetting agents also may be present.

The addition of the specified amount of one or more of the specified densifiers to a given ANFO mixture results in a composition having a higher detonation pressure than the given mixture, although substantially the same ideal detonation velocity. The ideal detonation velocity is the steady value attained at a sufiiciently long distance from the initiator in a tube or charge of diameter sufficiently large that further increase in either length or diameter will not cause an increase in velocity. The differences in density and detonation pressure computed at a given detonation velocity for a straight ANFO composition (no foreign densifier) and the same composition after the addition of Fe are shown below. The ANFO is a mixture of 94% AN and 6% fuel oil. All of the densities shown, except 0.785 g./cc., can be attained by blending coarse and fine AN. A density as low as 0.785 g./cc. is not likely to be obtained even with all-prill AN, but the detonation velocity computed for this density can be achieved by reducing the oil content to about 4.5%.

These results, as well as others which have been obtained, show that the detonating composition of this invention, which is produced by the detonation-pressureincreasing process of the invention, has a diiferent detonation pressure/velocity characteristic from that of straight ANFO compositions. As was stated previously, the detonation pressure, P, in kilobars, is approximately equal to the density, p, of the explosive or blasting agent times the square of the detonation velocity, D, in km./ sec. Furthermore, p is approximately equal to D- For a straight ANFO composition (94% AN, 6% fuel oil), P is approximately equal to 1.0 D On the other hand, for the composition of this invention, P is approximately equal to 1.1 D for a composition containing 10% Fe O 1.2 D for a composition containing 20% Fe 0 and 1.45 D for a composition containing 30% Fe O This pressure/velocity relationship shows that at any given detonation velocity, achieved by controlling the density, a higher detonation pressure is obtained as more densifying agent is added.

The ANFO detonating composition of this invention, which comprises AN in the size blends specified above as well as the previously specified amount of densifying agent, has a bulk density of at least about 1.00 gram per cubic centimeter, and an ideal detonation pressure, calculated for a composition containing 10% densifier, ranging from about 35 kilobars at a detonation velocity of 4000 m./sec. to about 60 kilobars at a detonation velocity of 5000 m./sec., these pressures and velocities being suitable for the diamond synthesis process described in US. Pat. 3,667,911, the disclosure of which patent is incorporated herein by reference. An improved diamond synthesis process employing the ANFO composition of the invention is described in the following examples.

Example 1 The procedure described in Example 5 of US. Pat. 3,667,911 is followed with certain exceptions. The sample tube is 167 inches long, and the driver tube 168 inches long. The graphite is in intimate mixture with copper as a cooling medium. The graphite/copper pellets are 2 /2 inches thick, and 44 pellets are packed into the sample tube to form a solid cylinder, in which the weight of graphite is 10,134 grams. The graphite density in the pellets is 50%. The top plug consists of one Z-inch-thick steel powder pellet pressed to a density of The bottom plug section consists of 27 steel powder pellets, each 2 inches thick, the density of the pellets varying from the innermost pellet to the outer pellet as follows: 74, 77, 80, 83, 86, 88, 90, 92, 90, 88, 86, 84, 82, 80, 75, 70, 65, 60, 55, 50, 45, 40, 40, 40, 40, 40, and 40%.

Surrounding and in contact with the driver tube is a 15.5-foot high cylinder 48 inches in diameter of a premixed detonating composition comprising 81% ammonium nitrate, 5% fuel oil, and 14% iron oxide (all by weight). The ammonium nitrate is a mixture of 60% whole prills (mean particle size 1.8 mm.) and 40% crushed prills (mean particle size 0.7-0.8 mm.). The iron oxide is -250 mesh (0.06-0.15 mm.), high-purity magnetic Fe O In loading, the detonating composition (13,050 pounds) is poured into an annular space between the driver tube and the surrounding steel culvert, and the culvert is hit with mallets to permit the composition to pack. The packed density of the composition is 1.14 g./ cc.

The cylindrical charge of detonating composition is initiated axially at the top end of the tube assembly by two coaxial HDP-l primers. The charge detonates at a velocity of 4520 meters per second (91.9% of the ideal velocity calculated for a density of 1.14 g./cc.). This composition detonates at a velocity of 3600 meters per second in a 9-inch diameter, and 4000 meters per second in an 18-inch diameter. Its computed ideal detonation pressure is 62 kilobars (50 kilobars at 4520 meters per second).

After detonation of the ANFO composition and recovery and separation of the carbon in the sample tube, diamond is obtained in 42.1% yield (weight of diamond recovered relative to the weight of carbon charged to the sample tube) and 50.9% conversion (weight of diamond recovered relative to the weight of carbon recovered after detonation).

Control Experiment-The procedure described in Example 1 is repeated with the exception that the detonating composition used is 10,500 pounds of a mixture of 95.5% ammonium nitrate prills (whole prills of 1.8 mm. mean particle size) and 4.5% of fuel oil. The density of this composition is 0.92 g./ cc. In a 48-inch diameter, the composition detonates at a velocity of 4540 meters per second. Its computed ideal detonation pressure is 51 kilobars (44 kilobars at 4540 meters per second). The diamond yield is 29.8%, and conversion 32.9%.

Example 2 The procedure described in Example 1 is repeated with the exception that the detonating composition contains 84% of the ammonium nitrate mixture, 6% fuel oil, 9% iron oxide, and 1% calcium stearate coating agent. Its weight is 13,850 pounds, its density 1.17 g./cc., and its detonation velocity in the 48-inch diameter 5050 meters per second (99% ideal velocity). The graphite density is 60%. The diamond yield is 60.8%, and conversion 64.0%

Example 3 Repetition of Example 1 with a detonating composition containing 76% of the ammonium nitrate mixture, 7% fuel oil, and 17% iron oxide (weight 13,600 pounds, density 1.15 g./cc., and detonation velocity in a 48-inch diameter 4415 meters per second), results in a diamond yield of 45.6%, and conversion of 56.8%.

Example 4 The following composition is also acceptable for use in the diamond synthesis described in the foregoing examples:

Percent Ammonium nitrate (60/40 whole/crushed prills) 80 Fuel oil 5 Crushed limestone 15 The packed density of this composition is 1.16 g./cc., and it detonates at a velocity of 3480 meters per second in a 9-inch diameter. The velocity in the 48-inch diameter used in the process is close to the ideal velocity of 4850 meters per second.

The ammonium nitrate prill and iron oxide mesh sizes specified herein are Tyler sieve sizes.

I claim:

1. A method of increasing the detonation pressure of a bulk ammonium nitrate/fuel oil composition substantially independently of its detonation velocity which comprises mixing (a) an ammonium nitrate/fuel oil compo sition comprised of about from 92 to 95 percent by weight of ammonium nitrate and about from 8 to 5 percent by weight of fuel oil and containing at least about 20 percent by weight of its ammonium nitrate in the form of prills, and (b) about from 8 to 30 percent by weight, based on the resulting mixture, of a particulate densifying agent selected from the group consisting of iron oxide, limestone, and anhydrite.

2. A method of Claim 1 wherein said densifying agent is iron oxide.

3. A method of Claim 2 wherein said ammonium nitrate is a blend of at least about 20 percent by weight of +20 mesh prills and at least about 20 percent by weight of a smaller-size grade up to about one-half the size of said prills, and the size grade of the iron oxide particles is up to about one-half the size of said prills.

4. A method of Claim 3 wherein said ammonium nitrate is a blend of about from 20 to 80 percent by weight of said prills and about from 80 to 20 percent by weight of said smaller-size grade.

5. A detonating composition comprising, in mixture:

(a) about from 64 to 87 percent by weight of discrete particles of ammonium nitrate in a larger-size and a smaller-size grade, said larger-size grade being defined by +20 mesh prills and constituting at least about 20 percent by weight of the ammonium nitrate particles in said mixture, and said smaller-size grade being up to about one-half the size of said prills and constituting at least about 20 percent by weight of the ammonium nitrate in said mixture;

(b) about from 8 to 30 percent by weight of discrete particles of a densifying agent selected from the group consisting of iron oxide, limestone, and anhydrite, the size grade of said particles of densifying agent being up to about one-half the size of said prills; and

(0) about from 3.5 to 8 percent by weight of fuel oil, the fuel oil weight in said mixture being about from 5 to 8 percent of the combined ammonium nitrate and fuel oil weight.

6. A detonating composition of Claim 5 wherein said densifying agent is iron oxide, the amount of said iron oxide in said mixture is about from 10 to 20 percent by weight of said mixture, and said ammonium nitrate is a blend of about from 20 to 80 percent by weight of said prills and about from 80 to 20 percent by weight of said smaller-size grade.

7. A detonating composition of Claim 6 wherein the amount of prills in said mixture is about from 40 to percent by weight of the ammonium nitrate therein, and said smaller-size grade is up to about one-fifth the size of said prills.

References Cited UNITED STATES PATENTS 3,540,953 11/1970 Schulze et a1 1492 3,180,768 4/1965 Scott 149-2 2,732,800 1/ 1956 Coursen 149-46 X 3,378,417 4/1968 McFerrin 14921 3,095,335 6/1963 McCloud et a1 14921 3,266,960 8/1966 Lyon et al 14946 X 3,212,944 10/1965 Lyon et a1 14946 X BENJAMIN R. PADGETT, Primary Examiner US. Cl. X.R. 1492, 45, 46, 112 

